PASADENA, Calif., Jan 5: As scientists search for signs of life on Mars and elsewhere in the solar system, they are sticking to one guiding principle - follow the water.

Indeed, NASA experts say their entire game plan for the latest stage of Mars exploration is based on sending twin robotic rovers, Spirit and Opportunity, to two spots on the red planet where the evidence of water is most compelling.

Spirit landed on Saturday night in the heart of Gusev Crater, a massive basin larger than Connecticut that was carved out by the impact of an asteroid or comet early in Mars' history and is thought to have once held a gigantic lake.

A long, deep valley apparently chiselled by ancient flows of water leads into Gusev, suggesting it was fed by an ancient river. In three weeks NASA plans to land Opportunity on the so-called Meridiani Planum, a smooth plain near Mars' equator where NASA's orbiting Mars Global Surveyor has detected an abundance of grey haematite, a mineral that on Earth usually forms in association with liquid water.

Some environmental conditions that can produce haematite, such as a lake or hot springs, could be hospitable to life. Others, such as hot lava, would not. 'We believe the pathway to finding life is to follow the water," said Firouz Naderi, Mars exploration programme manager.-Reuters